In general, a bill processing apparatus, which handles a bill as one of the embodiments of the paper sheet, is incorporated into a service device such as a game medium rental machine installed in a game hall, a vending machine or a ticket-vending machine installed in a public space, or the like which judges the authenticity of the bill inserted from a bill insertion slot by a user and provides various types of products and services in accordance with a value of the bill having been judged as authentic.
Normally, when a user inserts a bill from a bill insertion slot, the bill is conveyed through a traveling route by a conveyance mechanism including a conveyor roller etc. and passes through a reading sensor for reading bill information on the way such that an authenticity judgment process is conducted. Further, a plurality of sensors for detecting a moving bill are arranged in a frame that defines a traveling route through which the bill moves so that the position of the bill inserted from the bill insertion slot can be grasped at all times. For example, Patent Document 1 discloses a paper sheet processing apparatus capable of reliably detecting a paper sheet (bill) left in the traveling route by arranging a plurality of sensors for detecting the paper sheet along the traveling route and by setting the installation intervals between respective sensors in the traveling route direction, the installation interval between a traveling start end and the sensor installed nearest to the traveling start end in the traveling route direction, and the installation interval between a traveling back end and the sensor installed nearest to the traveling back end in the traveling route direction to a length equal to or less than the length of the paper sheet in the traveling direction. Further, Patent Document 1 discloses detecting the presence of the paper sheet in a state where the conveyance of the paper sheet is stopped, by way of example, in a state where the power source is just turned on or in a state where jamming is just cleared.